1. Field of the Invention
The present invention relates to an electron microscope equipped with an X-ray spectrometer.
2. Description of the Related Art
Heretofore, an electron probe microanalyzer (EPMA) comprising a scanning electron microscope (SEM) to which a wavelength-dispersive spectrometer (WDS) is attached has been known. In particular, when a specimen is irradiated with an electron beam within the electron microscope, characteristic X-rays are emitted. The emitted X-rays are detected by the WDS, and an X-ray analysis (elemental analysis) is made. This WDS needs a mechanism for aligning three points (X-ray generation point (specimen), the center point of a dispersive crystal, and the center point of the slit in the detector) to given points on a Rowland circle. Furthermore, the radius of the Rowland circle is as long as several meters. Consequently, a large-sized optical system arises. Furthermore, since the incidence angle to the dispersive crystal (i.e., the angle with respect to the normal line to a crystal face) is small, the detector is placed close to the microscope column of the electron microscope. This makes it difficult to determine the arrangement of the whole equipment.
Attempts have been made to combine an energy-dispersive spectrometer (EDS) with a transmission electron microscope (TEM) or SEM. Characteristic X-rays from a specimen are detected by the EDS. However, the EDS has unsatisfactory energy resolution compared with a WDS for electron probe microanalysis (EPMA).
We have developed a combination of a transmission electron microscope (TEM) and an energy filter to provide an instrument capable of making an energy analysis at a high resolution. Where this instrument is used, the dielectric function and the distribution of state densities of the conduction band of an area of a specimen having a diameter of 30 nm can be known. For detailed research into electron states, it is necessary to know the state density distribution in the valence band as well as the state density distribution in the conduction band.
As mentioned previously, a transmission electron microscope (TEM) equipped with an EDS can make an elemental analysis using characteristic X-rays produced from an area irradiated with an electron beam. If the spectrum of the characteristic X-rays can be measured at an energy resolution of better than about 1 eV, the distribution of state densities of the valence band can be known. Unfortunately, the energy resolution of the current EDS using a semiconductor detector is approximately 100 to 200 eV, which is insufficient for research into electron states. Furthermore, WDS has higher resolution (about 10 eV) than EDS but the energy resolution is not high enough to know the state densities of valence bands.
It is an object of the present invention to provide an electron microscope which is equipped with an X-ray spectrometer, has a compact optical system, and provides high energy resolution.
An electron microscope according to the present invention has an X-ray spectrometer mounted to the sidewall of the microscope via a gate valve. The X-ray spectrometer has a spectrometer chamber whose interior is evacuated by a vacuum pumping system. At least one diffraction grating having unequally spaced grooves is disposed in the chamber. An X-ray detector is mounted to one end of the chamber. Characteristic X-rays produced from a specimen irradiated with an electron beam are made to impinge on the face of the diffraction grating at a large angle with respect to the normal line to the face. That is, the X-rays are obliquely incident on the face. The diffracted X-rays are detected by the X-ray detector.
A back-illuminated CCD detector can be used as the aforementioned X-ray detector. Preferably, the exit angle of the X-rays diffracted by the diffraction grating having the unequally spaced grooves is 75 to 87xc2x0 with respect to the normal line to the face of the diffraction grating.
It is also possible to mount an X-ray condenser mirror for collecting the characteristic X-rays emitted from the specimen toward the diffraction grating having the unequally spaced grooves.
In another embodiment of the invention, a plurality of diffraction gratings each of which has unequally spaced grooves and which are different in measured energy range are mounted. A diffraction grating-exchanging mechanism can be provided which can selectively place the diffraction gratings in a characteristic X-ray incident position, one at a time.
In a further embodiment of the invention, a grating tilt-adjusting mechanism is provided which adjusts the tilt of the diffraction grating of unequally spaced grooves set in the characteristic X-ray incident position. Preferably, the CCD detector described above is connected with the spectrometer chamber via a bellows and thus the chamber is movable relative to the diffraction grating.
Preferably, the bellows described above consists of plural bellows elements cascaded together, the bellows elements stretching and contracting in different directions. The CCD detector is movable in two dimensions relative to the diffraction gratings by combining stretching and contraction of the bellows elements.
Other objects and features of the invention will appear in the course of the description thereof, which follows.